The influence of the doping concentration and reverse intersystem crossing on the efficiency of tricomponent organic light-emitting diodes with the thermally activated delayed fluorescence exciplex emitter

In this work, we fabricate a series of full-fluorescent organic light-emitting diodes (OLEDs) with the thermally activated delayed fluorescence (TADF) exciplex emitter in order to improve the efficiency through the reverse intersystem crossing (RISC) process. The TADF exciplex emitters are made up of a mixture of P-type materials (DMAC-DPS and mCBP) and n-type material (PO-T2T), among which DMAC-DPS also classes as a TADF material. The change in doping concentration will affect the intermolecular distance and the composition of TADF material and two kinds of exciplexes (DMAC-DPS:PO-T2T and mCBP:PO-T2T) in the luminescent layer (EML). Different materials and concentrations of doping not only add new RISC channels but also alter the original RISC channels, thereby affecting the performance of devices. It is beneficial for improving efficiency by increasing the proportion of independent TADF material and reducing the proportion of exciplex (DMAC-DPS:PO-T2T) in the EML, which can be controlled by doping. When the ratio of DMAC-DPS, PO-T2T and mCBP in the EML is 1 : 1 : 2, we achieve the optimal electro-optic performance in device A3, with maximum current efficiency, power efficiency, and luminance of 41.64 cd A−1, 43.42 lm W−1, and 23 080 cd m−2, respectively.


Introduction
2][3][4] For OLEDs based on conventional uorescent materials (FOLEDs), the maximum internal quantum efficiency (IQE) is limited to 25% as only singlet excitons can be utilized. 57][8][9] In recent years, non-conventional uorescent materials such as the thermally activated delayed uorescence (TADF) material and exciplex forming co-hosts have attracted tremendous attention and effort from both academia and industry. 9,10Among them, low-energy triplet excitons can obtain energy to up-convert to the emissive singlet level through the endothermic reverse intersystem crossing (RISC) process. 11 2012, Adachi et al. reported a class of metal-free organic electroluminescent molecules in which the energy gap between the singlet and triplet excited states was minimized by design, and these simple aromatic compounds exhibited efficient TADF with high photoluminescence efficiency. 12Adachi and coworkers also rst demonstrated the TADF characteristics of exciplexes and achieved an external quantum efficiency (EQE) of 5.4% by using a blend exciplex (m-MTDATA:3TPYMB) as emitter. 13Compared with single-molecule TADF emitters, TADF exciplex emitters do not need complicated molecule design and synthesis, and just require suitable electron-donating (D) and electron-accepting (A) constituent molecules. 9Lee et al. presented a practical and straightforward approach to improve the emission performance of the TADF type exciplex by dispersing the exciplex (DMAC-DPS:PO-T2T) in the TADF host (DMAC-DPS).The EQE of the exciplex device was improved to 15.3% by adopting the emitting layer (EML) structure with low n-type material (PO-T2T) content because of the suppressed exciton quenching by spatial separation of the exciplex in EML. 14 In this article, the author used three materials to create two RISC channels.Zhang et al. proposed a strategy of building TADF exciplex emitters with three components to realize three RISC channels respectively on DBT-SADF, DBT-SADF:PO-T2T, and CDBP:PO-T2T, which can effectively improve the exciton utilization in TADF exciplex emitters. 15The author attributed the increased RISC process to their reduced energy gap (DE ST ), which can be realized by separating the spatial distance of the electron-hole pairs.However, they did not provide a detailed discussion on the proportion of three components in EML and the role of each RISC channel.

Experimental
The indium-tin oxide (ITO) layer is precoated onto the glass substrate.The glass substrates with a surface resistivity of 15 U sq. −1 are sequentially cleaned with cleaning agent solution, deionized water, ethanol, and isopropanol, each step lasting for 10 minutes, followed by drying and cooling in a drying oven for 30 minutes.All devices are prepared by vacuum deposition at a pressure below 5 × 10 −4 Pa.During the fabricating process, all organic materials are grown on substrates at a rate of 0.01-1 Å s −1 .The evaporation rate of the doping material is adjusted based on the doping concentration.The electro-optical characteristics and EL spectra of all devices are measured by using a Keithley 2400 source meter and a PR655 spectroradiometer, respectively.All the measurements are carried out in the ambient atmosphere.

Results and discussion
Firstly, we investigate the effect of the doping concentration (x) of p-type material mCBP in EML on electro-optical and spectral characteristics in series of device A, which is based on device A1 with a TADF exciplex emitter (DMAC-DPS:PO-T2T).As indicated in Fig. 1(b), the values of x in devices A1-A4 varies, corresponding to 0%, 25%, 50% and 75%, respectively.Fig. 2(a) shows the luminance-voltage-current density characteristic curves of devices A1-A4.Device A2 achieves the maximum luminance of 25 830 cd m −2 among all four devices, but its efficiencies are lower than that of devices A3 and A4 as shown in Fig. 2(b) and Table 1.As the doping concentration of mCBP increases, the current efficiency and power efficiency of the device are gradually improving.Although the device A4 with the highest doping concentration of 75% has a maximum current efficiency of 42.55 cd A −1 and a maximum power efficiency of 43.95 lm W −1 , its maximum luminance is only 12 880 cd m −2 .In device A3, the efficiency and luminance of the device are relatively high, with the current efficiency, power efficiency, and luminance being 41.64 cd A −1 , 43.42 lm W −1 , and 23 080 cd m −2 , respectively.The detailed values of EL characteristic for all devices tested with different structures are listed in Table 1.
The relative spatial distance between donors and acceptors in exciplex systems can affect the potential energy surfaces of ground states and excited states, which in turn affects the emission characteristics of the device. 15,23Yuan et al. improve electro-optical characteristics of TCTA:PO-T2T exciplex-based OLEDs by co-doping an organic spacer (mCP) into the bulkheterojunction exciplex emitter (TCTA:PO-T2T) because of the long-range coupling of the electron-hole pairs.They believe that the increased RISC process from the triplet to singlet states is due to their reduced energy gap (DE ST ). 15,24,25Besides the reduced DE ST , we regard that the changes of the RISC channel caused by doping and dispersing molecules are also another important reason for the remarkable improvement in device performance.As shown in Fig. 3, there are only two RISC channels (process I and process III) resulting from TADF material DMAC-DPS and exciplex material DMAC-DPS:PO-T2T in device A1, respectively.The doping of p-type material mCBP increases the distance between molecules DMAC-DPS and PO-T2T and makes their density more dispersed in EML, which reduces the probability of forming exciplex (DMAC-DPS:PO-T2T).7][28] As the doping concentration of mCBP increases, the probability of forming the independent TADF material (DMAC-DPS) and the exciplex (mCBP:PO-T2T) in EML increases signicantly, which results in an increase in the Because the HOMO energy level of mCBP is higher than that of DMAC-DPS as shown in Fig. 1(a), the excessive mCBP doping makes the current injection more difficult.The current density of device A4 gradually decreases compared to that of device A3, resulting in a decrease in luminance from 23 080 cd m −2 to 12 880 cd m −2 .Fig. 2(c) indicates the normalized EL spectra of devices A1-A4 at 11.5 V voltage.In device A1 without mCBP doping, the main peak wavelength of the EL spectrum at 536 nm mainly comes from the energy release of the singlet energy level (2.32 eV) of the exciplex material DMAC-DPS:PO-T2T. 26The TADF material DMAC-DPS has a singlet energy level of 2.9 eV, corresponding to the main peak position of 520 nm in the EL spectra. 27As the doping concentration increases, the spectral peak values of devices A1-A4 exhibit a slight blue shi, with the color coordinate ranging from (0.370, 0.566) to (0.287, 0.536).This blue shi indicates that the change in the ratio of exciplexes and TADF material has led to a shi in the main RISC process of excitons from process I and process III to process I and process II.As the voltage changes from 3.5 V to 11.5 V, the spectral variation of device A3 is little as shown in Fig. 2(d).
Secondly, we investigate the effect of the doping concentration (y) of the n-type material PO-T2T on the electro-optical and spectral characteristics of device B prepared on the basis of device A3.As indicated in Fig. 1(b), the values of y in devices B1, A3, B2 and B3 varies, corresponding to 0%, 25%, 50% and 75%, respectively.Fig. 4(a), (b) and Table 1 show the electro-optical characteristic curves and experimental data of the devices B1, A3, B2 and B3.Compared with three B devices, device A3 has the highest maximum current efficiency, power efficiency, and luminance, with values of 41.64 cd A −1 , 43.42 lm W −1 , and 23 080 cd m −2 , respectively.In device B1, there is only a unique RISC channel (process I) resulting from TADF material DMAC-DPS in EML, as there is no exciplex due to the concentration of PO-T2T being 0. As the doping concentration of PO-T2T increases, the distance between PO-T2T molecules and DMAC-DPS and mCBP molecules also decreases, which makes it easier to form exciplex mCBP:PO-T2T and DMAC-DPS:PO-T2T.As the content of PO-T2T gradually increases from 0, the RISC channel in process II and process III begins to appear.At low doping concentration, multiple RISC channels can cause more excitons to up-convert to the emissive singlet state of TADF material, thereby improving the maximum power efficiency to 43.42 lm W −1 of device A3.However, as the doping content of PO-T2T further increases, the probability of forming exciplex (DMAC-DPS:PO-T2T) increases and the probability of the independent TADF material decreases signicantly.This will signicantly weaken the role of RISC channel of process I, and the maximum power efficiency of the device will begin to decrease to 23.93 lm W −1 of device B3.At the same time, the position of the main peak in the normalized EL spectra shis from (0.267, 0.499) to (0.372, 0.564) with increasing the doping concentration of PO-T2T as shown in Fig. 5(a).This also indicates that the luminescence resulting from two exciplexes (mCBP:PO-T2T, DMAC-DPS:PO-T2T) has become the main source process.The luminescence of TADF material almost disappears in devices B2 and B3.
Finally, we investigate the effect of the doping concentration (z) of the TADF material DMAC-DPS on the electro-optical and spectral characteristics of device C prepared on the basis of device A3.As indicated in Fig. 1(b), the values of z in devices C1, A3, C2 and C3 varies, corresponding to 0%, 25%, 50% and 75%, respectively.Fig. 4(c), (d) and Table 1 show the electro-optical    PO-T2T.The corresponding effect of process I also weakens, while that of process II and III gradually strengthens.Therefore, the device efficiencies of devices B and C show a signicant decrease compared to that of device A3 as high concentration doping occurs.A gradual red shi of the normalized EL spectra corresponding to a shi in exciton energy from the singlet energy level of DMAC-DPS to that of DMAC-DPS:PO-T2T is observed as the doping concentration (z) of DMAC-DPS increased from 0% to 75% as shown in Fig. 5(b).

Conclusion
In summary, we realize a change of the energy transfer path in a series of tricomponent OLEDs by doping different materials into TADF exciplex emitters.We ultimately achieve the best device performance in device A3, with maximum current efficiency, power efficiency, and luminance of 41.64 cd A −1 , 43.42 lm W −1 , and 23 080 cd m −2 , respectively.These electro-optical characteristics far exceed the performance of conventional full-uorescent OLEDs device.When low doping content in devices A, B, and C, the utilization efficiency of excitons in EML is improved due to the increase of RISC channels, thereby improving the device efficiency.When the doping concentration is greater than 25% in device B and C, the number of independent TADF molecules in EML decreases due to the decrease in intermolecular distance, and the RISC channel caused by TADF material weakens, resulting in a signicant decrease in efficiency of devices B and C. The proportion between TADF material (DMAC-DPS) and two kinds of exciplexes (DMAC-DPS:PO-T2T and mCBP:PO-T2T) is closely related to the channels of RISC, and different channels affect the utilization efficiency of excitons in EML.The spectral shi with doping concentration in devices A, B and C is consistent with the change of RISC channels.

Fig. 1
Fig. 1 (a) The energy level diagrams of all materials used in our tricomponent OLEDs; (b) schematic diagrams in EML of fabricated devices A, B and C by doping.

Fig. 2
Fig. 2 (a) Current density and luminance versus voltage curves of devices A1-A4; (b) power efficiency and current efficiency versus current density curves of devices A1-A4; (c) the normalized EL spectra of devices A1-A4 at 11.5 V driving voltage; (d) the normalized EL spectra of device A3 at different voltages from 3.5 V to 11.5 V.

Fig. 3
Fig. 3 Schematic diagram of energy transfer including RISC channels in EML of device A, B and C.

Fig. 4
Fig. 4 (a) Current density and luminance versus voltage curves of devices B1, A3, B2 and B3; (b) power efficiency and current efficiency versus current density curves of devices B1, A3, B2 and B3; (c) current density and luminance versus voltage curves of devices C1, A3, C2 and C3; (d) power efficiency and current efficiency versus current density curves of devices C1, A3, C2 and C3.

Table 1
Electro-optical characteristics of all devices tested with different structures The maximum current density.b The maximum luminance.c The maximum current efficiency.d The maximum power efficiency. a

Table 2
Comparison of the efficiency with other reported OLED with the same exciplex The maximum current density.b The maximum luminance.c The maximum current efficiency.d The maximum power efficiency. a